When manufacturing aircraft parts, for example, a section of a wing box upper cover, the upper cover is manufactured with high tolerances. In particular, controlling the surface waviness, the surface finish and the steps and gaps between interfaces of such an upper cover are all important factors because these factors may result in aircraft parts that have poor aerodynamics.
A known method for manufacturing wing covers and similar parts involves co-curing stiffeners to a flat panel to create a monolithic part. However, this method has disadvantages associated with it, such as spring-in and distortion of the part during the curing process, as discussed in more detail below. The distortion can be reduced using standard known techniques. However, for some designs the distortion tolerance is too high for such standard techniques.
FIG. 1 illustrates a typical wing cover 2 in cross section on the left and viewed from above on the right. The wing cover 2 is manufactured from carbon fiber reinforced polymer (CFRP) and includes a CFRP wing skin panel 4, stiffened using CFRP T-section stiffeners or stringers 6. The T-section stiffeners 6 are made up of a horizontal or base portion (or foot) 10 and an upright or vertical portion (or blade) 8. The upright portion 8 is substantially perpendicular to the base portion 10.
FIG. 2 illustrates in cross section a typical T-section stiffener 6. The stiffener includes an upright portion 8 and a base portion 10. The stiffener shown in the figure is produced using a single piece of CFRP that has been roll-formed using known techniques to produce the stiffener shown.
FIG. 3 shows an alternative known stiffener 12 that has been manufactured using two L-section stiffeners bonded together along a bond-line 14 using epoxy, for example, to form a T-shape stiffener.
During the manufacture of components using T-section stiffeners attached to the components, there is a tendency for the component to spring or distort during the cure process. FIG. 4 illustrates a roll-formed stringer or stiffener 6 being brought into contact with a panel or skin 4 to be bonded.
FIG. 5 illustrates a roll formed stiffener 6 with spring-in. T-section stiffeners are known to spring in, due to thermal expansion and the radius of curvature between the foot or horizontal portion 10 and the blade or vertical/upright portion 8. In the figure the spring-in of the foot 10 is illustrated by numeral 10a showing the foot 10 before it is co-cured with no spring-in and 10b after it is co-cured with spring-in. The spring-in angle 16 is illustrated on the figure, but it will be appreciated that the amount of spring-in may be determined by subtracting angle 16 from 90 degrees (i.e., the angle before spring-in). When the skin panel 4 (FIG. 4) and the roll-formed stiffener 6 are co-cured, the spring induced by the stiffener or stringer 6 is transferred into the panel 4, as illustrated in FIG. 6. The effect is that the skin panel 4 is pulled in the direction indicated by arrows 18 in the figure. This pull-in results in surface waviness on the outer mold line of the wing skin. It will be appreciated that the spring-in illustrated in FIGS. 5 and 6 has been exaggerated for illustrative purposes, but is typically between 1 and 5 degrees.
The cause of spring-in is generally attributed to the coefficient of thermal expansion (CTE) of the composite part (e.g. the stiffener), and the CTE properties being orthotropic (i.e., vary in the 3 different axial directions). Indeed, spring-in is observed when the stiffener is unconstrained, as illustrated in FIG. 5.
FIG. 7 illustrates an L-section stiffener or stringer 20 before being cured (left hand side) and after being cured (right hand side). The stiffener 20 shown in the figure illustrates the spring-in of the foot or horizontal portion 22 toward the blade or upright portion 24 of the stiffener 20. The amount of spring-in for each of the foot and blade 22, 24 is illustrated as angle “x” in the figure, such that a total spring-in angle of 2x is observed.
If two L-section stiffeners are positioned back to back or with their respective blades adjacent one another and joined along bond-line 26 (i.e., co-cured, co-bonded or otherwise) to each other to form a T-section, the spring-in that would be experienced by one blade 24a of the L-section stiffener 20a is resisted by the spring-in of the opposing blade 24b of the L-section stiffener 20b. As a result, each blade 24a, 24b remains in its original, upright, position, and it is the stiffener feet 22a, 22b that move upwards to an angle of 2x, as illustrated in FIG. 8. This is observed for T-section stiffeners which are roll-formed or stiffeners formed using back-to-back L-sections.
As discussed above, when a T-section stiffener is co-cured to a flat panel, the movement of the stiffener feet or horizontal portions is transferred to the flat panel, thus pulling the panel in the same direction as the stiffener movement. The effect of the stiffener feet movement may be observed as a profile deviation, or waviness, on the outer mould line face of the flat panel, or for a wing cover, the aerodynamic surface of the cover may be outside allowed tolerances.
Therefore, there is a need to design a stiffener that minimises the amount of spring-in that is transferred to the panel or wing cover by the stiffener. Further, the stiffener preferably should have a single flange or contact surface for attachment to the wing skin.